Immune checkpoint blockade of programmed death 1 (PD-1) and its ligand 1 (PD-L1) has dramatically increased progression-free survival for many cancers (1-3). For example, a monoclonal antibody (mAb) drug, pembrolizumab (Keytruda®), received regulatory approval for use in patients with microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) solid tumors (4, 5). While mAb drugs inhibiting immune checkpoints are highly useful in oncology, there is a desire for other types of inhibitors of of immune checkpoints, such as small-molecules. Small-molecule drugs targeting PD-1 may lead to increases in efficacy and safety of cancer treatment, as well to improved access to cancer treatments.